The purification of liquid has become of great interest in many industries. In particular, pure water is used for many industrial purposes rather than merely as drinking water. For example, pure water is used in processes for producing semiconductor chips, in power plants, in the petro chemical industry and for many other purposes.
Ion exchange resins, reverse osmosis filtration and electrodialysis techniques have been used to reduce the concentration of particular ions in a liquid.
Electrodeionization apparatus have recently been used with more frequency to reduce the concentration of ions in a liquid. The term "electrodeionization" generally refers to an apparatus and a process for purifying liquids which combine ion exchange resins, ion exchange membranes and electricity to purify the liquids. An electrodeionization module comprises alternating arrangements of cation permeable membranes and anion permeable membranes defining compartments therebetween. In alternating compartments, there is provided ion exchange resin beads. Those compartments are known as diluting compartments. The compartments which generally do not contain ion exchange resin are known as the concentrating compartments. Ions migrate from the diluting compartments through ion exchange beads and ion permeable membranes into the concentrating compartments by the introduction of current. The liquid flowing through the concentrating compartments is discarded or partially recycled and the purified liquid flowing through the diluting compartments is recovered as demineralized liquid product.
Electrodialysis apparatus are similar in configuration to electrodeionization apparatus. The main difference between electrodialysis apparatus and electrodeionization apparatus is that electrodialysis apparatus do not use ion exchange resin to aid in the removal of ions in the liquid passed through the diluting compartment. Often electrodialysis apparatus utilize membrane structures extending into the diluting compartments to aid in the removal of ions from a liquid.
There are two general configurations for electrodeionization and electrodialysis apparatus: first, a plate and frame configuration, and second, a spiral-wound configuration.
U.S. Pat. No. 4,925,541 which issued May 15, 1990 to Giuffrida et al. discloses a plate and frame electrodeionization apparatus and method. The method for removing ions from a liquid in an electrodeionization apparatus is carried out in an electrodeionization apparatus which has a number of subcompartments in the diluting compartments. A mixture of anion exchange resin and cation exchange resin is contained within the subcompartments. The subcompartments are formed by a plurality of ribs extending along the length of the diluting or ion depletion compartments.
U.S. Pat. No. 4,636,296 which issued Jan. 13, 1987 to Kunz discloses another embodiment of plate and frame apparatus and method for the demineralization of aqueous solutions in which an aqueous liquid is passed through alternating separate layers of cation exchange resin and anion exchange resin.
Plate and frame apparatus are large in size and typically suffer from leaks because of the difficulty of sealing large vessels. Also, the units often are oversize because of inflexibility in designing for capacity, necessitating undesirably high capital and operating costs.
U.S. Pat. No. 5,376,253 which issued Dec. 27, 1994 to Rychen et al. discloses an apparatus for the electrochemical desalination of aqueous solutions. The apparatus has a wound or spiral arrangement of anion and cation permeable membranes. Such apparatus are prone to leakage and are relatively difficult to manufacture.
It is tedious to increase or vary the total output capacity of purified liquid for plate and frame configurations because it involves disassembly, insertion of additional ion permeable membranes, and installation of longer tie-bars to assemble the apparatus together. It is also tedious if not impossible to increase or vary the total output capacity of purified liquid for spiral configurations because it involves disassembly and the insertion of a longer or shorter arrangement of anion and cation permeable membranes.
It is desirable to easily vary the total output capacity for pure liquid in apparatus for the demineralization of liquids. It is also desired to have an electrochemical cell for electrodialysis and electrodeionization apparatus which is relatively easy to situate in an existing water treatment system.